The present invention is directed to a process for the diagnosis and prevention of supravalvular aortic stenosis (SVAS) and Williams syndrome. SVAS and Williams syndrome is diagnosed in accordance with the present invention by analyzing the DNA sequence of the elastin gene of an individual to be tested and comparing the DNA sequence to the known DNA sequence of a normal elastin gene. Alternatively, the elastin gene of an individual to be tested can be screened for mutations associated with SVAS or Williams syndrome. Prediction of SVAS and Williams syndrome will enable practitioners to prevent these disdorders using existing medical therapy.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference and for convenience are respectively grouped in the appended List of References.
Supravalvular aortic stenosis (SVAS) is an inherited vascular disorder (1). As its name implies, narrowing of the ascending aorta is a dominant feature of this disease, but other arteries, including the pulmonary arteries, may be affected. If uncorrected, SVAS may lead to increased intracardiac pressure, myocardial hypertrophy, heart failure and death. The incidence of SVAS is estimated to be 1 in 25,000 live births. The vascular abnormalities typical of SVAS can be inherited as an isolated, autosomal dominant trait (1-3) or as part of a second disease, Williams syndrome, a developmental disorder that affects multiple organ systems (2-4). In addition to vascular disease, manifestations of Williams syndrome include hypertension, mental retardation, an unusually gregarious personality, premature greying of the hair, premature aging of the skin, joint laxity early in life followed by joint contractures, dysmorphic facial features and infantile hypercalcemia (4). The relationship between SVAS and Williams syndrome was previously undefined. Occasionally patients with Williams syndrome have been noted in families with SVAS.
SVAS was first described in 1842 (5), but the pathogenesis of this disorder was unknown until now. Mechanistic hypotheses have been based on clinical and pathological studies, but these data are conflicting. It is not clear, for example, whether hypertrophy or hyperplasia of medial smooth muscle is the more prominent feature of this disorder. O'Conner et al. (6) examined tissue from six individuals with SVAS; two of these cases were familial, one was a sporadic case of SVAS, and three had Williams syndrome with SVAS. These investigators did not discover any significant pathologic differences between the individuals with different SVAS inheritance patterns. They noted that the medial layer of the aorta in all patients showed a haphazard arrangement of thick elastic fibers, excessive collagen, hypertrophied smooth-muscle cells, and scant ground substance. This contrasts with normal medial tissue, which is highly organized and arranged in parallel layers of connective tissue and smooth muscle. They also observed that smooth muscle cells formed clumps or bundles and were the major component of the medial layer. In a study of a single individual with SVAS, Perou also showed that the diseased media contained excessive smooth muscle (7). The resulting pattern was that of irregular fascicles of smooth muscle surrounded by fibrous and collagenous tissue. In contrast to these studies, Pober et al. recently reported a study of seven individuals with SVAS and Williams syndrome, noting that the medial layer of affected aortas contained an increased number of smooth muscle cells and normal to decreased collagen. These investigators also observed an elevated level of platelet-derived growth factor (PDGF) and concluded that increased quantities of PDGF stimulate smooth muscle proliferation and cause the cardiovascular abnormalities of SVAS (8).
The pathogenic mechanisms underlying Williams syndrome are unknown, but many hypotheses have invoked a mechanism of abnormal calcium metabolism (9). These hypotheses are based on the intermittent observation of infantile hypercalcemia in Williams syndrome as well as studies showing that excessive vitamin D administration can produce pathologic changes in the supravalvular aortic wall of rabbits during development (10). Recent attempts to repeat this work, however, have failed and the hypothesis involving vitamin D has been questioned (11). Researchers have tried to identify a causal relationship between calcitonin-gene-related peptide (CGRP) and Williams syndrome. Hitman et al. investigated 13 families with Williams syndrome and infantile hypercalcemia for germ line mutations in the CGRP gene, and found no mutations (12). Using linkage analysis, investigators have also excluded the CGRP gene, which is located on chromosome 11, in a family with autosomal dominant SVAS (13).
Diagnosis of SVAS has been based on family history, physical examination and echocardiography. Unfortunately, these tests may be ambiguous, making early detection of this disorder difficult. The recent advent of high quality two-dimensional and color flow Doppler echocardiography have improved non-invasive screening for SVAS (14), but invasive tests such as cardiac catheterization and angiography are more sensitive. Currently, vascular surgery is the only treatment option for SVAS. Therefore, it is an object of the present invention to provide better diagnostic methods for screening for SVAS and Williams syndrome which will ultimately lead to new medical therapies for their prevention.